This invention relates to a fuse-programmable decoder for selecting redundant memory cells in a semiconductor memory device such as a dynamic random-access memory, static random-access memory, electrically erasable and programmable read-only memory, or other type of memory.
As the integration density of memory devices has increased, it has become necessary to reduce power dissipation by dividing the memory cell array into multiple sections, only one of which operates in any given access cycle. To improve production yields, each section is provided with redundant memory cells. If a defective memory cell is found during testing by the manufacturer, the device is programmed so that when the defective cell is addressed, a redundant memory cell will be accessed instead.
For this purpose, the device is provided with a fuse-programmable decoder to decode address signals and select redundant memory cells when pre-programmed addresses appear. The decoder operates by precharging a node to the power-supply potential, then allowing the node to discharge through a network of transistors and fusible links, referred to herein as a fuse-programmable ROM, if the address does not match a pre-programmed address. A separate node and fuse-programmable ROM are provided for each section of the memory device. (ROM stands for road-only memory.)
A problem with existing fuse-programmable decoders is that even though only one section of the memory cell array will be selected, in every memory access cycle al decoder nodes are precharged, and all the nodes corresponding to non-selected sections must be discharged. This leads to unnecessary current drain and power dissipation.
A further problem is that when a node is discharged, the source and drain electrodes of transistors coupled to that node are both placed at the ground potential. These transistors become equivalent to single capacitors coupled between the address signal lines and ground, placing a large capacitive load on the address signal lines, which slows memory operation and leads to more unnecessary power dissipation.